Add define that pulls in reporter and listeners interfaces

This allows users to define reporters and listeners in files different
from the main file.

Related to #991, #986

include/catch.hpp

@@ -19,6 +19,7 @@
 
 #if defined(CATCH_CONFIG_MAIN) || defined(CATCH_CONFIG_RUNNER)
 #  define CATCH_IMPL
+#  define CATCH_CONFIG_EXTERNAL_INTERFACES
 #endif
 
 #ifdef CATCH_IMPL
@@ -52,6 +53,10 @@
 #include "internal/catch_objc.hpp"
 #endif
 
+#ifdef CATCH_CONFIG_EXTERNAL_INTERFACES
+#include "internal/catch_external_interfaces.h"
+#endif
+
 #ifdef CATCH_IMPL
 #include "internal/catch_impl.hpp"
 #endif